The present invention generally relates to a gas detecting device, and in particular to a gas detecting device suitable for a gas leak alarm designed to detect gas such as LP gas and commercialized gas.
A gas detecting device is proposed in the Japanese Laid-Open Patent Application No. 61-191953. The proposed gas detecting device (hereinafter referred to as a gas detector) is illustrated in FIG. 1A. The gas detector includes a substrate 11, an insulator layer 12, a pair of detection leads 13, an insulation coating layer 14, and a gas sensitive layer 15. The gas sensitive layer 15 is formed so as to partially overlie the insulator layer 12, the detection leads 13 and the insulation coating layer 14. The gas sensitive layer 15 is formed by depositing a metal oxide (ceramics) semiconductor material such as a tin dioxide,(SnO.sub.2), ferrous oxide (Fe.sub.2 O.sub.3), and zinc oxide (ZnO) by evaporation, sputtering, chemical vapor deposition, ion plating, and so on. The gas detecting operation is performed by applying heat to the gas sensitive layer 15 through a heater layer (not shown).
However, the proposed gas detector has disadvantages described below. The gas sensitive layer 15 is formed by depositing ceramics at low temperature. Therefore, volumetric shrinkage occurs in the gas sensitive layer 15. The ratio of shrinkage for the insulator layer 12, the detection leads 13 and the insulation coating layer 14 is smaller than that for the gas sensitive layer 15, even if those layers are grown at low temperatures.
In a case where the heater layer 13 detection lead is made of platinum (Pt), and the insulator layer 12 and the insulation coating layer 14 are made of silicon dioxide, the thermal expansion ratio for the detector lead is greater than those for the insulator layer 12 and the insulation coating layer 14, which are far greater than that for the gas sensitive layer 15. Thus, dislocation (or shear) is liable to occur in an interface between the gas sensitive layer 15 and the other layers. As a result, as shown in FIG. 1B, the gas sensitive layer 15 is liable to come off the gas detector.
In addition, the gas sensitive layer 15 is formed in a great step portion formed by the detection leads 13 and the insulation coating layer 14. For this reason, as shown in FIG. 2, the gas sensitive layer 15 is made thin at portions P, and may be cracked. That is, the step coverage of the gas sensitive layer 15 is poor. The above event degrades gas detection characteristics.